Assembly fixture with anisotropic thermal properties

ABSTRACT

In one embodiment, a method includes fastening a plurality of components of a composite structure in an assembly fixture, wherein the assembly fixture comprises a plurality of strands of a fiber-reinforced thermoplastic material, wherein the fiber-reinforced thermoplastic material comprises a thermoplastic embedded with a plurality of reinforcement fibers, wherein the plurality of reinforcement fibers is aligned within each strand of the plurality of strands, and wherein the assembly fixture further comprises an anisotropic thermal expansion property based on an orientation of the plurality of reinforcement fibers within the assembly fixture; and heating the assembly fixture in an autoclave to bond the plurality of components of the composite structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional and claims the benefit of priorityunder 35 U.S.C. § 120 of U.S. application Ser. No. 16/814,622, filed onMar. 10, 2020, entitled “ASSEMBLY FIXTURE WITH ANISOTROPIC THERMALPROPERTIES,” Inventors David G. Carlson et al., which application is adivisional and claims the benefit of priority under 35 U.S.C. § 120 ofU.S. application Ser. No. 15/710,797, filed on Sep. 20, 2017, entitled“ASSEMBLY FIXTURE WITH ANISOTROPIC THERMAL PROPERTIES,” Inventors DavidG. Carlson et al., issued as U.S. Pat. No. 10,654,208 B2 on May 19,2020. The disclosure of the prior applications are considered part ofand are incorporated in their entirety by reference in the disclosure ofthe applications.

TECHNICAL FIELD

This disclosure relates generally to composite structures, and moreparticularly, though not exclusively, to tooling used for manufacturingcomposite structures.

BACKGROUND

Aircraft are often designed using composite structures. A compositestructure, for example, may include a combination of different materialsintegrated together to achieve certain structural properties.Manufacturing a composite structure, however, may require specializedmanufacturing tooling to be designed and built for the particularcomposite structure. Moreover, building the tooling used to manufacturea composite structure can be time-consuming and expensive.

SUMMARY

According to one aspect of the present disclosure, a method may comprisefastening a plurality of components of a composite structure in anassembly fixture, wherein the assembly fixture comprises a plurality ofstrands of a fiber-reinforced thermoplastic material, wherein thefiber-reinforced thermoplastic material comprises a thermoplasticembedded with a plurality of reinforcement fibers, wherein the pluralityof reinforcement fibers is aligned within each strand of the pluralityof strands, and wherein the assembly fixture further comprises ananisotropic thermal expansion property based on an orientation of theplurality of reinforcement fibers within the assembly fixture; andheating the assembly fixture in an autoclave to bond the plurality ofcomponents of the composite structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example rotorcraft in accordance with certainembodiments.

FIGS. 2A-D illustrate an example embodiment of an anisotropic assemblyfixture.

FIGS. 3A-F illustrate an example embodiment of an anisotropic mold.

FIG. 4 illustrates a flowchart for additively manufacturing a toolingcomponent.

DETAILED DESCRIPTION

The following disclosure describes various illustrative embodiments andexamples for implementing the features and functionality of the presentdisclosure. While particular components, arrangements, and/or featuresare described below in connection with various example embodiments,these are merely examples used to simplify the present disclosure andare not intended to be limiting. It will of course be appreciated thatin the development of any actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, including compliance with system, business,and/or legal constraints, which may vary from one implementation toanother. Moreover, it will be appreciated that, while such a developmenteffort might be complex and time-consuming, it would nevertheless be aroutine undertaking for those of ordinary skill in the art having thebenefit of this disclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as depicted in the attached drawings. However, aswill be recognized by those skilled in the art after a complete readingof the present disclosure, the devices, components, members,apparatuses, etc. described herein may be positioned in any desiredorientation. Thus, the use of terms such as “above,” “below,” “upper,”“lower,” or other similar terms to describe a spatial relationshipbetween various components or to describe the spatial orientation ofaspects of such components, should be understood to describe a relativerelationship between the components or a spatial orientation of aspectsof such components, respectively, as the components described herein maybe oriented in any desired direction. Further, the present disclosuremay repeat reference numerals and/or letters in the various examples.This repetition is for the purpose of simplicity and clarity and doesnot in itself dictate a relationship between the various embodimentsand/or configurations discussed.

Example embodiments that may be used to implement the features andfunctionality of this disclosure will now be described with moreparticular reference to the attached FIGURES.

FIG. 1 illustrates an example embodiment of a rotorcraft 100. Rotorcraft100 includes a fuselage 110, a main rotor system 120, and an empennage130. The fuselage 110 is the main body of rotorcraft 100, and mayinclude a cabin (e.g., for crew, passengers, and/or cargo) and/or housecertain mechanical and electrical components (e.g., engine(s),transmission, and/or flight controls). The rotor system 120 ofrotorcraft 100 is used to generate lift. For example, the rotor system120 includes a plurality of rotor blades 122 a-d that rotate when torqueis supplied by the engine(s), which in turn generates lift. Moreover,the pitch of each rotor blade 122 can be adjusted in order toselectively control direction, thrust, and lift for rotorcraft 100.Rotorcraft 100 also includes a tail rotor system 140 to provideanti-torque and/or directional control, which similarly includes aplurality of rotating rotor blades 142 a-d, and is positioned on theempennage 130. The empennage 130 further includes a horizontalstabilizer 132 and a vertical stabilizer 134, which respectively providehorizontal and vertical stability for rotorcraft 100.

The components of an aircraft, such as rotorcraft 100, are oftendesigned using composite structures. A composite structure, for example,may include a combination of different materials integrated together toachieve certain structural properties, which are typically superior tothe properties of the underlying materials individually. For example, acomposite structure may be lightweight yet relatively strong, renderingit particularly suitable for aircraft and/or other applications whereweight and/or strength are critical to performance. Accordingly,composite structures are often used in the design of wings, rotorblades, engine blades, propellers, flight control surfaces, fairings,spoilers, stabilizers, airframe structural components, fuselages,various interior components (e.g., floors, walls, fixtures), and soforth. Manufacturing a composite structure, however, may requirespecialized manufacturing tooling to be designed and built for theparticular composite structure. Moreover, building the tooling used tomanufacture a composite structure can be time-consuming and expensive.

As an example, a wing or blade for a rotor system often includes avariety of composite components, such as spars, ribs, skin(s), and soforth. These underlying components are often created separately and thensubsequently bonded together. For example, the underlying components maybe fabricated separately using various molds, and may then be bondedtogether using an assembly fixture. The fabrication process for aparticular component, for example, may involve placing an associatedmold in an autoclave to cure the component at an elevated pressure andtemperature (e.g., 250+ degrees Fahrenheit). The resulting componentsmay then be bonded together in an assembly fixture, which can beaccomplished at either room temperature or an elevated temperaturedepending on the particular bonding approach used. In some cases, forexample, the components may be bonded together in an assembly fixture atroom temperature using fasteners and/or paste adhesives. In other cases,however, the components may be bonded together in the assembly fixtureat an elevated pressure and temperature (e.g., in an autoclave) usingthermoset resins or adhesives. A thermoset resin, for example, is aresin that undergoes an irreversible chemical transformation from atacky to rigid state when cured at high temperatures, and thus can onlybe cured once. Compared to room temperature bonding (e.g., usingfasteners and/or paste adhesives), high-temperature bonding usingthermoset resins may result in a stronger bond and may eliminate and/orreduce the need for mechanical fasteners.

Fabricating and bonding composite components at high temperatures,however, requires the associated molds and assembly fixture(s) to havecertain thermal properties, such as suitable thermal expansionproperties. For example, when subjected to high temperatures, a materialmay expand at a particular rate in various directions. The rate at whicha material expands in a given direction is represented by itscoefficient of thermal expansion (CTE). A material that has the same CTEin all directions may be referred to as isotropic, while a material thatdoes not have the same CTE in all directions may be referred to asanisotropic. Many composite components are designed using isotropicmaterials with a low CTE in order to minimize expansion when cured athigh temperatures. The tooling used to cure the composite components(e.g., assembly fixtures and bond molds) may be similarly designed withappropriate CTE properties to minimize abnormalities and deformationduring the curing process. For example, if the CTE properties of thetooling differ meaningfully from those of the composite component beingcured, the tooling may stretch and/or compress the composite componentduring the curing process (e.g., as the temperature rises and falls),which may lead to manufacturing abnormalities and/or tearing or crushingof the composite component. For example, if an assembly fixture has ahigh CTE, the assembly fixture may expand as its temperature risesduring the curing process, causing the resulting component to similarlystretch, grow, and/or tear, and the assembly fixture may subsequentlycontract after the curing process finishes and its temperature lowers,causing the resulting component to be compressed and/or crushed.Accordingly, in some cases, the tooling used for high-temperature curingof a composite component may be designed using an isotropic materialwith a low CTE that roughly matches the CTE of the composite componentitself. In some cases, for example, the tooling may be designed usingInvar, an isotropic nickel-iron alloy with a low CTE. The process ofbuilding Invar-based tooling, however, can be very time-consuming andexpensive.

Accordingly, this disclosure describes various embodiments of tooling(e.g., assembly fixtures and molds) that can be built efficiently and issuitable for high-temperature curing of composite components. Forexample, this disclosure describes embodiments of additive-manufacturedthermal-anisotropic tooling, which can be built using additivemanufacturing and tailored with optimal anisotropic thermal properties.The ability to build the tooling using additive manufacturingsignificantly reduces the time and expense typically required to buildtooling used for high-temperature curing (e.g., Invar-based tooling).Moreover, the ability to tailor anisotropic thermal properties of thetooling allows its thermal expansion properties to be optimized forhigh-temperature curing. For example, the tooling can be designed withdifferent CTEs in different dimensions to minimize expansion in certaindirections, provide compression bonding in certain directions, and/orfacilitate extraction of the resulting composite component from thetooling. For example, the tooling can be designed with a low CTE incertain dimension(s) where minimizing thermal expansion is critical toavoiding manufacturing abnormalities. The tooling can also be designedwith a CTE in certain dimension(s) that facilitates bonding by exertingcompressive forces in certain directions. Finally, the tooling can alsobe designed with a CTE in certain dimension(s) that facilitatesextraction of the composite component from the tooling, for example, bycausing the tooling to expand during the curing process so that itsubsequently contracts and releases itself from the composite componentafter the curing process is complete.

In some embodiments, for example, the tooling may be built from afiber-reinforced thermoplastic resin using additive manufacturing. Forexample, unlike a thermoset resin that can only be cured once, athermoplastic resin does not undergo an irreversible chemicaltransformation when heated at high temperatures, and thus can be curedmultiple times. Thus, while a composite component itself may be formedusing a thermoset resin that can only be cured once, the associatedtooling is formed using a thermoplastic resin that can be curedrepeatedly. In this manner, the tooling can be built using additivemanufacturing to repeatedly extract and cure layers of thefiber-reinforced thermoplastic resin. Moreover, the additivemanufacturing process can be strategically performed in order to alignthe reinforcement fibers in a manner that achieves desired anisotropicthermal properties. For example, a thermoplastic resin typically has ahigh CTE, while fiber reinforcements typically have a low CTE.Accordingly, the fiber reinforcements can be aligned in certaindirection(s) in the thermoplastic matrix in order to achieve desired CTEproperties for certain dimensions of the tooling.

In this manner, the tooling can be built using additive manufacturing,while also allowing its thermal properties to be tailored with optimalCTE values for high-temperature curing. In addition, the flexibility ofthis additive manufacturing approach enables the resulting tooling toachieve an appropriate stiffness while remaining lightweight, thusimproving transportation and handling of the tooling, and facilitatingrapid heating and cooling of the tooling due to its lower thermal mass.Moreover, the ability to use additive manufacturing significantlyreduces the time and expense required to build tooling with suitablethermal expansion properties for high-temperature curing, thussignificantly reducing the cycle time for manufacturing compositecomponents. For example, while Invar-based tooling may take months oryears to build, tooling designed using the embodiments of thisdisclosure may only take day(s) or week(s) to build. Accordingly, thedescribed embodiments can be used to significantly reduce the cycle timefor manufacturing composite components. In some embodiments, forexample, the described embodiments may be used to efficiently createtooling and reduce the manufacturing cycle time for aircraft components,such as wings, rotor blades, engine blades, propellers, flight controlsurfaces, fairings, spoilers, stabilizers, airframe structuralcomponents, fuselages, various interior components (e.g., floors, walls,fixtures), and so forth. The described embodiments can also be used forother applications, such as the manufacture of land-based vehicles(e.g., cars, buses, trucks), water-based vehicles (e.g., boats,submarines), spacecraft, and/or any other applications involvingcomposite components.

Example embodiments are described below with more particular referenceto the remaining FIGURES. It should be appreciated that rotorcraft 100of FIG. 1 is merely illustrative of a variety of aircraft that can beused with embodiments described throughout this disclosure. Otheraircraft implementations can include, for example, fixed wing airplanes,hybrid aircraft, tiltrotor aircraft, unmanned aircraft, gyrocopters, avariety of helicopter configurations, and drones, among other examples.Moreover, the described embodiments can also be used for othernon-aircraft implementations, including land, water, and/or space-basedvehicles, among other examples.

FIGS. 2A-D illustrate an example embodiment of an anisotropic assemblyfixture 200. Assembly fixture 200 can be used to assemble a compositestructure using high-temperature bonding of the underlying components.Moreover, assembly fixture 200 can be built using additive manufacturingand tailored with anisotropic thermal expansion properties that aresuitable for high-temperature bonding. The ability to use additivemanufacturing to build assembly fixture 200 enables it to be builtsignificantly faster and cheaper than other types of high-temperaturebond assembly fixtures (e.g., Invar-based assembly fixtures), thussignificantly reducing the manufacturing cycle time for a particularcomponent. Moreover, the ability to tailor the anisotropic thermalexpansion properties of assembly fixture 200 allows them to be optimizedfor high-temperature bonding. Accordingly, unlike other types ofassembly fixtures, assembly fixture 200 can be built in a time- andcost-efficient manner, while still achieving appropriate thermalexpansion properties for high-temperature bonding. In the illustratedembodiment, assembly fixture 200 is designed for assembling a blade orwing 230 for the tail rotor of a rotorcraft (e.g., tail rotor 140 ofrotorcraft 100 from FIG. 1). In other embodiments, however, an assemblyfixture for any other type of component can be designed and built usinga similar approach.

In the illustrated embodiment, assembly fixture 200 can be used toassemble the underlying components of rotor wing 230 by bonding themtogether using high-temperature bonding. For example, wing 230 is formedusing a variety of components, including spars 232 a-c, ribs 234 a-d,and a skin 236, along with various other components omitted from theillustrated example for simplicity. In some cases, these underlyingcomponents may be created separately (e.g., using a mold) and thensubsequently bonded together using assembly fixture 200. Assemblyfixture 200 is used to securely hold the underlying components of wing230 in the appropriate positions during the bonding process. Forexample, assembly fixture 200 includes a base structure 210 and aplurality of detail fixtures 220 a-c extending from the base, which arecollectively used to securely position the various components of wing230, such as spars 232 and ribs 234. In some cases, the variouscomponents of wing 230 may then be bonded together usinghigh-temperature bonding. For example, after securing the underlyingcomponents of wing 230 in assembly fixture 200, assembly fixture 200 maybe placed in an autoclave to bond the components together using bondadhesives (e.g., thermoset resins) that are cured at an elevatedpressure and temperature (e.g., 250+ degrees Fahrenheit).

Moreover, assembly fixture 200 can be built using additive manufacturingin a manner that allows its thermal expansion properties to be optimizedfor high-temperature bonding. For example, growth that results fromthermal expansion may vary across different dimensions of a structuredepending on the CTE and size of each dimension. In particular, growthin a particular dimension increases with the magnitude of the CTE forthat dimension, and also increases with the size of the dimension. Thus,even dimensions with the same CTE will experience varying levels ofgrowth if they differ in size. As an example, wing 230 has a length thatis significantly larger than its height and width, and assembly fixture200 has dimensions with similar proportions in order to accommodate thewing 230. Thus, wing 230 and assembly fixture 200 are much moresusceptible to thermal growth across their length than their height andwidth. The components of wing 230, however, are typically designed usingisotropic materials with a low coefficient of thermal expansion (CTE) inorder to minimize thermal expansion across all dimensions to avoidmanufacturing abnormalities. However, if the CTE properties of assemblyfixture 200 differ meaningfully from those of wing 230, thermalexpansion may cause assembly tooling 200 to produce manufacturingabnormalities or defects in wing 230 during the bonding process. Forexample, as assembly fixture 200 thermally expands during the bondingprocess due to its rising temperature, assembly fixture 200 may stretchand/or tear wing 230 if they expand at meaningfully different rates.Similarly, as assembly fixture 200 subsequently contracts after thebonding process completes and its temperature lowers, assembly fixture200 may compress and/or crush wing 230. While assembly fixture 200 couldbe designed using Invar (an isotropic material with a low CTE) so thatit roughly matches the CTE of wing 230 in all dimensions in order tominimize manufacturing abnormalities, the process of buildingInvar-based tooling can be very time-consuming and expensive. Moreover,it may be sufficient for assembly fixture 200 to only match the CTE ofwing 230 in the length dimension, as that dimension is significantlylarger and thus more susceptible to thermal growth than the otherdimensions.

Accordingly, assembly fixture 200 is designed using a material that canbe additively manufactured with tailored anisotropic thermal properties,thus allowing assembly fixture 200 to be built in a fast andcost-efficient manner, and also tailored with a low CTE in the lengthdimension to minimize thermal growth that might otherwise subject wing230 to manufacturing abnormalities. In some embodiments, for example,assembly tooling 200 may be built using an anisotropic material 240formed as a fiber-reinforced thermoplastic matrix. For example, as shownin FIG. 2D, anisotropic material 240 may include a thermoplastic resinor matrix 216 impregnated with fibers 217. A thermoplastic matrix 216,for example, is a plastic material or polymer that becomes pliable ormoldable at high temperatures and solidifies upon cooling. In someembodiments, for example, thermoplastic matrix 216 may include acrylic(polymethyl methacrylate), acrylonitrile butadiene styrene, nylon,polylactic acid (polylactide), polybenzimidazole, polycarbonate,polyether sulfone, polyoxymethylene, polyetherether ketone,polyetherimide, polyethylene, polyphenylene oxide, polyphenylenesulfide, polypropylene, polystyrene, polyvinyl chloride, and/or teflon,among other examples. Moreover, in some embodiments, fibers 217 mayinclude carbon fibers, graphite fibers, glass fibers, and/or any othersuitable type of reinforcement fiber. Prior to the additivemanufacturing process, the fibers 217 of anisotropic material 240 arerandomly arranged within thermoplastic matrix 216. During the additivemanufacturing process, however, the anisotropic material 240 can beextruded in a manner that causes fibers 217 to become aligned in aparticular direction within thermoplastic matrix 216. Moreover, becausethermoplastic matrix 216 typically has a high CTE while reinforcementfibers 217 typically have a low CTE, the resulting structure createdusing this approach will have a low CTE in dimension(s) that correspondwith the directional alignment of fibers 217, and a high CTE in theremaining dimensions. Accordingly, the additive manufacturing processcan be performed strategically in order to align the reinforcementfibers 217 in a manner that achieves desired anisotropic thermalproperties in the resulting assembly fixture 200.

For example, as depicted by FIGS. 2B and 2C, assembly fixture 200 can beadditively manufactured using fused deposition modeling (FDM) to extrudeanisotropic material 240 in layers or strands 215 in the lengthwisedirection to form the base 210 of assembly fixture 200. As the layers orstrands 215 of anisotropic material 240 are extruded through a nozzle,the fibers 217 become aligned in the direction in which the layers orstrands 215 are extruded. In this manner, the resulting base structure210 for assembly fixture 200 has a low CTE in the lengthwise dimension,but a high CTE in the height and width dimensions (e.g., as shown bygraph 250 of the CTE properties of base 210). Moreover, while a low CTEis important for the length of base 210, a high CTE is acceptable forits height and width, as those dimensions are significantly smaller andthus much less susceptible to thermal growth.

The detail fixtures 220 a-c of assembly fixture 200 can be additivelymanufactured in a similar manner, except the anisotropic material 240may be extruded in a manner that achieves a low CTE in the heightdimension of each detail fixture 200, given that the height of a detailfixture 220 is its largest dimension and thus is more susceptible tothermal growth than its width or depth. In some embodiments, forexample, each detail fixture 220 may be additively manufactured byextruding anisotropic material 240 in layers or strands 215 that areoriented along its height.

Finally, in some embodiments, the thermal properties of assembly fixture200 can be further tailored in order to leverage thermal expansion forcompression bonding purposes. For example, assembly fixture 200 can bedesigned so that its CTE in the lengthwise dimension of its base 210 isslightly smaller than the CTE of wing 230. In this manner, assemblyfixture 200 will thermally expand at a slightly slower rate than wing230 in the lengthwise dimension, creating pressure between wing 230 andassembly fixture 200 as they press against each other, and thusproducing a compressive force that aids the bonding process.Accordingly, this approach may reduce or eliminate the need to usemechanical pressurizers as aids during the bonding process. In someembodiments, for example, tailoring the lengthwise CTE of assemblyfixture 200 for compression bonding purposes can be achieved by formingthe anisotropic material 240 using appropriate types and quantities ofunderlying materials, and/or extruding anisotropic material 240 in anappropriate arrangement during the additive manufacturing process. Forexample, anisotropic material 240 can be formed by selecting athermoplastic matrix 216 and fibers 217 with appropriate CTE properties,and/or adjusting the density of fibers 217 within thermoplastic matrix216 to alter the resulting CTE properties of anisotropic material 240.

In this manner, assembly fixture 200 can be additively manufactured withtailored anisotropic thermal properties, thus allowing assembly fixture200 to be built in a fast and cost-efficient manner and with suitablethermal expansion properties for high-temperature bonding of thecomponents of wing 230. Accordingly, the manufacturing cycle time forwing 230 can be significantly reduced, as the requisite tooling can bebuilt significantly faster than is possible for existing types oftooling.

FIGS. 3A-F illustrate an example embodiment of an anisotropic mold 300.Mold 300 can be used, for example, to fabricate a composite componentusing high-temperature curing. Moreover, mold 300 may be created usingadditive manufacturing and tailored anisotropic thermal properties, thusallowing mold 300 to be built in a fast and cost-efficient manner andwith suitable thermal expansion properties for high-temperature curingof a composite component. In the illustrated embodiment, mold 300 isdesigned to fabricate a spar 332 that can be used in the assembly of awing or blade for the tail rotor of a rotorcraft. In some embodiments,for example, spar 332 fabricated by mold 300 may be subsequently used byassembly fixture 200 from FIG. 2 for the assembly of a tail rotor wing230. In other embodiments, however, mold 300 may be designed tofabricate any other type of composite component.

In the illustrated embodiment, spar 332 is fabricated by shaping athermoset composite spar material 330 in mold 300, and then curing sparmaterial 330 at a high temperature and pressure (e.g., using anautoclave). The spar material 330, for example, may be afiber-reinforced thermoset matrix, such as a thermoset resin impregnatedwith reinforcements fibers. A thermoset resin, for example, is a resinthat undergoes an irreversible chemical transformation from a tacky torigid state when cured at high temperatures. Moreover, reinforcementfibers can be impregnated in the thermoset resin using an orientationdesigned to achieve desired structural properties for spar 332. In someembodiments, for example, spar material 330 may be carbon epoxy, whichis an epoxy thermoset resin impregnated with carbon fibers. In otherembodiments, however, spar material 330 may include any other type ofthermoset resin and/or reinforcement fibers.

Accordingly, spar material 330 is first molded into to a spar shapeusing mold 300. For example, mold 300 includes an outer mold line (OML)tool 310 and an inner mold line (IML) tool 320. OML tooling 310 is usedto shape the outer surface of the spar, while IML tooling 320 is used toshape the inner surface of the spar. IML tooling 320 is initiallyseparate from OML tooling 310 (as shown by FIG. 3A), allowing sparmaterial 330 to be laid up on IML tooling 320 in numerous layers to formthe shape of the spar (as shown by FIG. 3B). IML tooling 320 is thenflipped over and inserted into OML tooling 310 (as shown by FIG. 3C). Inthis manner, after spar material 330 is shaped appropriately on IMLtooling 320, it is subsequently compressed between OML tooling 310 andIML tooling 320, as shown by FIGS. 3D and 3E. Spar material 330 can thenbe cured in mold 300 at a high temperature and pressure (e.g., in anautoclave), causing it to transform from a tacky to rigid state, thusforming the resulting spar 332 (as shown by FIG. 3F).

Moreover, mold 300 can be designed with thermal expansion propertiesthat are optimal for high-temperature curing of a spar. In someembodiments, for example, OML tooling 310 may be designed with a low CTEin its width dimension, while IML tooling 320 may be designed with ahigh CTE in its width dimension. In this manner, spar material 330 canbe initially shaped on IML tooling 320 in a smaller form than the finalresulting spar 332, allowing the IML tooling 320 with the shaped sparmaterial 330 to be easily inserted into OML tooling 310. Moreover,during the curing process, the CTE differential for the width dimensionof OML tooling 310 and IML tooling 320 causes spar material 330 tostretch to its full size, while also being pressurized or compactedbetween the respective tooling components. For example, the high CTE forthe width of IML tooling 320 causes it to thermally expand, thusstretching spar material 330 to its full width until it reaches thewalls of OML tooling 310, and OML tooling 310 then limits any furtherexpansion due to its low CTE. In this manner, spar material 330 ispressurized or compacted between OML tooling 310 and IML tooling 320,which aids in achieving a well-formed spar 332 at the completion of thecuring process. Moreover, when the curing process completes, the widthdimension of IML tooling 320 subsequently contracts as its temperaturereturns to normal, thus releasing itself from the resulting spar 332. Inthis manner, the high CTE for the width of IML tooling 320 also servesto automatically extract the resulting spar 332 from mold 300, thuseliminating the need for a separate extraction step.

Moreover, it may also be desirable to design OML tooling 310 and IMLtooling 320 with a low CTE in the length dimension in order to minimizemanufacturing abnormalities during fabrication of the spar material 330.For example, because OML tooling 310 and IML tooling 320 aresignificantly larger in length than in height or width, they are muchmore susceptible to thermal growth across their length, which can causethem to stretch or tear the resulting spar 332 as they thermally expandduring the curing process, and/or crush spar 332 as they subsequentlycontract when the curing process completes. Accordingly, designing OMLtooling 310 and IML tooling 320 with a low CTE in the length dimensionminimizes thermal expansion in that dimension, thus reducing thelikelihood of manufacturing abnormalities in the resulting spar 332.

While OML tooling 310 and IML tooling 320 could be designed usingisotropic thermal materials, exclusively using isotropic materials is aninflexible approach that does not allow the optimal CTE to be achievedfor all dimensions of both tooling components. For example, as notedabove, it may be desirable for OML tooling 310 and IML tooling 320 tohave different CTEs in their width dimensions, but a low CTE in theirlength dimensions. Accordingly, the desired CTE differential in thewidth dimension could be achieved by designing OML tooling 310 using anisotropic material with a low CTE (e.g., Invar), while designing IMLtooling 320 using an isotropic material with a high CTE (e.g.,aluminum). However, while a high isotropic CTE for IML tooling 320 isdesirable for its width dimension (e.g. to achieve the appropriate CTEdifferential between the respective tooling components), it isundesirable for its length dimension, as the high CTE subjects IMLtooling 320 to thermal expansion in the length dimension, which canproduce manufacturing abnormalities in the fabricated spar 332. On theother hand, if IML tooling 320 is designed using an isotropic materialwith a low CTE (e.g., Invar) instead of a high CTE, its low isotropicCTE is desirable for its length dimension to minimize thermal expansion,but undesirable for its width dimension given that the respectivetooling components would no longer have a CTE differential in the widthdimension. Accordingly, if OML tooling 310 and IML tooling 320 aredesigned exclusively using isotropic materials, they can either bothhave a low CTE in all directions in order to minimize thermal expansion,or they can have a mismatched CTE in all directions in order to leveragethe benefits of the CTE differential in the width dimension, but bothadvantages cannot be achieved simultaneously.

Accordingly, in some embodiments, IML tooling 320 can be additivelymanufactured in a manner that allows it to have tailored anisotropicthermal expansion properties, thus allowing it to be built in a fast andcost-efficient manner and with optimal thermal expansion properties ineach dimension. In some embodiments, for example, IML tooling 320 may bebuilt using an anisotropic material formed from a fiber-reinforcedthermoplastic matrix, using a similar process described in connectionwith assembly fixture 200 from FIG. 2. For example, the anisotropicmaterial may include a thermoplastic resin or matrix impregnated withreinforcement fibers. Prior to the additive manufacturing process, thefibers of the anisotropic material may be randomly arranged within thethermoplastic matrix. During the additive manufacturing process,however, the anisotropic material may be extruded in a manner thatcauses the fibers to become aligned in a particular direction within thethermoplastic matrix. Moreover, because a thermoplastic matrix typicallyhas a high CTE while reinforcement fibers typically have a low CTE, theresulting structure created using this approach will have a low CTE indimension(s) that correspond with the directional alignment of fibers,and a high CTE in the remaining dimension(s). Accordingly, IML tooling320 can be additively manufactured in a strategic manner that aligns thereinforcement fibers appropriately in order to achieve the desiredanisotropic thermal expansion properties.

For example, IML tooling 320 can be additively manufactured using fuseddeposition modeling (FDM) to extrude strands or layers of theanisotropic material through a nozzle in an arrangement that ultimatelyforms the IML tooling component 320. Moreover, as strands of theanisotropic material are extruded through the nozzle, the fibers in theanisotropic material become aligned in the direction in which thestrands are extruded. Accordingly, when additively manufacturing IMLtooling 320, the strands of anisotropic material can be extruded with alengthwise orientation, thus achieving a low CTE in the length dimensionand a high CTE in the other dimensions (e.g., as shown by graph 350 ofthe CTE properties of IML tooling 320). In this manner, IML tooling 320achieves the desired low CTE for its length dimension, and the desiredhigh CTE for its width dimension.

Accordingly, in some embodiments, OML tooling 310 can still be designedusing a low CTE isotropic material such as Invar, as the appropriate CTEproperties for mold 300 can be achieved when isotropic OML tooling 310is used in conjunction with anisotropic IML tooling 320. In otherembodiments, however, OML tooling 310 can be designed using a similaradditive manufacturing process as IML tooling 320, except with strandsof the fiber-reinforced thermoplastic material extruded in anarrangement that achieves a low CTE in at least the length and widthdimensions.

Finally, a mold for any other type of composite component can be createdin a similar manner to that described for mold 300, for example, byprocessing an appropriate a composite material (e.g., fiber-reinforcedthermoplastic and/or any other suitable composite material) in a mannerthat achieves the desired thermal expansion properties. As one example,the described approach could be used to create a mold for an aircraftskin stringer, which is a component used to fasten the skin of anaircraft. The described approach could also be used to create a mold foran I-beam, for example, tailored with a low CTE in the length dimensionand a high CTE in the height dimension (e.g., to minimize thermal growthin the length dimension, while leveraging thermal growth in the heightdimension to facilitate extraction of the resulting I-beam from themold).

FIG. 4 illustrates a flowchart 400 for additively manufacturing atooling component. In some embodiments, for example, flowchart 400 maybe used to additively manufacture a tooling component used for themanufacture of a composite structure, such as an assembly fixture or amold used to manufacture an aircraft component at an elevatedtemperature (e.g., assembly fixture 200 of FIG. 2 and/or mold 300 ofFIG. 3).

The flowchart may begin at block 402 by forming a composite materialthat comprises a thermoplastic resin and a plurality of reinforcementfibers. The composite material can be formed, for example, byimpregnating a thermoplastic resin with a large number of fibers. Insome embodiments, the composite material may be formed as small pelletsof thermoplastic resin with embedded fibers. Moreover, when thecomposite material is initially formed, the arrangement of fibers withinthe thermoplastic resin is random.

In other embodiments, however, the composite material may be formedusing any other combination of underlying materials, depending on thedesired structural and/or thermal properties. For example, the compositematerial could be formed using rubber, metal (e.g., aluminum), concrete,plywood, carbon, glass, graphite, and/or ceramic, among other examples.

The flowchart may then proceed to block 404 to heat the compositematerial to transform it into a viscous form. For example, because thecomposite material contains a thermoplastic resin, heating the compositematerial transforms it into a viscous form that can be shaped or molded.

The flowchart may then proceed to block 406 to extrude a plurality ofstrands of the composite material. For example, after the compositematerial has been heated into a viscous form, strands of the compositematerial can be extruded through a nozzle. In some embodiments, forexample, a pellet of the composite material can be extruded through thenozzle into a strand. Moreover, in some embodiments, fused depositionmodeling can be used to extrude strands of the composite materialthrough the nozzle. In this manner, while the initial arrangement offibers within the composite material is random, extruding a particularstrand of the composite material causes the reinforcement fibers withinthat strand to align in the direction in which the strand is extruded ororiented.

In other embodiments, however, the composite material may be processedusing any other suitable manufacturing techniques, such as melting,pouring, and/or roll forming, among other examples.

The flowchart may then proceed to block 408 to arrange the strands ofcomposite material in a manner that forms a tooling component withanisotropic thermal expansion properties.

When extruding strands of the composite material, for example, thestrands can be arranged in a manner that ultimately forms a particularstructure or component. For example, a desired structure or componentcan be additively manufactured (e.g., using fused deposition modeling)by extruding strands of the composite material in an appropriatearrangement.

Moreover, the thermal expansion properties of the resulting structurecan be tailored based on the orientation of the strands used to form thestructure. For example, a thermoplastic typically has a high CTE, whilereinforcement fibers typically have a low CTE. As noted above, however,extruding a strand of the composite material causes the reinforcementfibers to align inside the thermoplastic in the direction that thestrand is extruded or oriented. Accordingly, the low CTE of the fibersprevents the thermoplastic from thermally expanding at its normal ratein the direction that the fibers are aligned. In this manner, anextruded strand has a lower CTE in the direction that the fibers arealigned, as the fibers serve to hold the thermoplastic together in thatdirection.

Accordingly, the thermal expansion properties of the resulting structurecan be tailored by arranging the strands in appropriate orientations. Insome embodiments, for example, the strands used to form the resultingstructure can be oriented in a manner that achieves a low CTE in certaindimension(s) and/or a high CTE in other dimension(s). In some cases, forexample, the strands may all be oriented in the same direction toachieve a low CTE for a particular dimension of the structure, and/or ahigh CTE for the remaining dimensions of the structure. Alternatively,the strands may be oriented in a variety of directions to achieve a lowCTE for multiple dimensions of the structure, and/or a high CTE for anyremaining dimension of the structure. In some embodiments, for example,the orientation of the strands may alternate within a layer and/orbetween layers of the resulting structure.

In this manner, any type of tooling component can be additivelymanufactured with optimal thermal expansion properties. In someembodiments, for example, the described approach can be used toadditively manufacture assembly fixtures and molds used for creatingcomposite aircraft components. The ability to additively manufacture atooling component significantly reduces the time and expense required tobuild the tooling component, while the ability to tailor the toolingcomponent with anisotropic thermal expansion properties allows it to beoptimized for fabricating and/or bonding a composite structure at anelevated temperature. In some embodiments, for example, the tooling canbe designed with different CTEs in different dimensions to minimizethermal expansion in certain directions, exert pressure in certaindirections (e.g., to facilitate compression bonding), and/or leveragethermal contraction to extract the resulting composite component fromthe tooling.

At this point, the flowchart may be complete. In some embodiments,however, the flowchart may restart and/or certain blocks may berepeated.

The flowcharts and diagrams in the FIGURES illustrate the architecture,functionality, and operation of possible implementations of variousembodiments of the present disclosure. It should also be noted that, insome alternative implementations, the function(s) associated with aparticular block may occur out of the order specified in the FIGURES.For example, two blocks shown in succession may, in fact, be executedsubstantially concurrently, or the blocks may sometimes be executed inthe reverse order or alternative orders, depending upon thefunctionality involved.

Although several embodiments have been illustrated and described indetail, numerous other changes, substitutions, variations, alterations,and/or modifications are possible without departing from the spirit andscope of the present invention, as defined by the appended claims. Theparticular embodiments described herein are illustrative only, and maybe modified and practiced in different but equivalent manners, as wouldbe apparent to those of ordinary skill in the art having the benefit ofthe teachings herein. Those of ordinary skill in the art wouldappreciate that the present disclosure may be readily used as a basisfor designing or modifying other embodiments for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. For example, certain embodiments may be implementedusing more, less, and/or other components than those described herein.Moreover, in certain embodiments, some components may be implementedseparately, consolidated into one or more integrated components, and/oromitted. Similarly, methods associated with certain embodiments may beimplemented using more, less, and/or other steps than those describedherein, and their steps may be performed in any suitable order.

Numerous other changes, substitutions, variations, alterations, andmodifications may be ascertained to one of ordinary skill in the art andit is intended that the present disclosure encompass all such changes,substitutions, variations, alterations, and modifications as fallingwithin the scope of the appended claims.

In order to assist the United States Patent and Trademark Office(USPTO), and any readers of any patent issued on this application, ininterpreting the claims appended hereto, it is noted that: (a) Applicantdoes not intend any of the appended claims to invoke paragraph (f) of 35U.S.C. § 112, as it exists on the date of the filing hereof, unless thewords “means for” or “steps for” are explicitly used in the particularclaims; and (b) Applicant does not intend, by any statement in thespecification, to limit this disclosure in any way that is not otherwiseexpressly reflected in the appended claims.

What is claimed is:
 1. A method, comprising: fastening a plurality ofcomponents of a composite structure in an assembly fixture, wherein theassembly fixture comprises a plurality of strands of a fiber-reinforcedthermoplastic material, wherein the fiber-reinforced thermoplasticmaterial comprises a thermoplastic embedded with a plurality ofreinforcement fibers, wherein the plurality of reinforcement fibers isaligned within each strand of the plurality of strands, and wherein theassembly fixture further comprises an anisotropic thermal expansionproperty based on an orientation of the plurality of reinforcementfibers within the assembly fixture; and heating the assembly fixture inan autoclave to bond the plurality of components of the compositestructure.
 2. The method of claim 1, further comprising additivelymanufacturing the assembly fixture.
 3. The method of claim 2, whereinthe additively manufacturing the assembly fixture comprises arrangingthe plurality of strands of the fiber-reinforced thermoplastic materialto form the assembly fixture.
 4. The method of claim 2, wherein theadditively manufacturing the assembly fixture comprises forming theassembly fixture using fused deposition modeling.
 5. The method of claim4, wherein the forming the assembly fixture using fused depositionmodeling further comprises extruding the plurality of strands of thefiber-reinforced thermoplastic material.
 6. The method of claim 1,wherein the composite structure comprises an airfoil.
 7. The method ofclaim 6, wherein the airfoil comprises an aircraft wing.
 8. The methodof claim 1, wherein the composite structure comprises a rotor blade. 9.The method of claim 1, wherein the anisotropic thermal expansionproperty of the assembly fixture comprises a coefficient of thermalexpansion for a length dimension of the assembly fixture lengthdimension that is smaller than a coefficient of thermal expansion for alength dimension of the composite structure.
 10. The method of claim 9,wherein the assembly fixture comprises a plurality of fasteningstructures coupled to a base structure, wherein the plurality offastening structures are configured to secure a plurality of componentsof the composite structure during the heated bonding process.
 11. Themethod of claim 10, wherein the anisotropic thermal expansion propertyof the assembly fixture comprises a low coefficient of thermal expansionfor a length dimension of the base structure and a height dimension ofthe plurality of fastening structures.
 12. A method of manufacturing acomposite structure using an assembly fixture comprising a plurality ofstrands of composite material, the method comprising: fastening aplurality of components of a composite structure in the assembly fixturehaving an anisotropic thermal expansion property based on an orientationof a plurality of reinforcement fibers comprising the compositematerial; and heating the assembly fixture in an autoclave to bond theplurality of components of the composite structure.
 13. The method ofclaim 12, wherein the composite material comprises fiber-reinforcedthermoplastic material, wherein the fiber-reinforced thermoplasticmaterial comprises a thermoplastic embedded with the plurality ofreinforcement fibers.
 14. The method of claim 13, wherein the pluralityof reinforcement fibers are aligned within the plurality of strands. 15.The method of claim 12, further comprising additively manufacturing theassembly fixture, the additively manufacturing the assembly fixturecomprising at least one of arranging the plurality of strands of thecomposite material to form the assembly fixture and using fuseddeposition modeling by extruding the plurality of strands of thecomposite material.
 16. The method of claim 12, wherein the compositestructure comprises at least one of an airfoil, an aircraft wing, and arotor blade.
 17. The method of claim 12, wherein the anisotropic thermalexpansion property of the assembly fixture comprises a coefficient ofthermal expansion for a length dimension of the assembly fixture lengthdimension that is smaller than a coefficient of thermal expansion for alength dimension of the composite structure.
 18. The method of claim 17,wherein the assembly fixture comprises a plurality of fasteningstructures coupled to a base structure, wherein the plurality offastening structures are configured to secure a plurality of componentsof the composite structure during the heated bonding process.
 19. Themethod of claim 18, wherein the anisotropic thermal expansion propertyof the assembly fixture comprises a low coefficient of thermal expansionfor a length dimension of the base structure and a height dimension ofthe plurality of fastening structures.
 20. A method, comprising: heatinga composite material into a viscous form, wherein the composite materialcomprises a thermoplastic and a plurality of reinforcement fibers,wherein the plurality of reinforcement fibers is randomly arrangedwithin the thermoplastic; extruding a plurality of strands of thecomposite material, wherein extruding the plurality of strands causesthe plurality of reinforcement fibers within each strand to align; andarranging the plurality of strands of the composite material to form anassembly fixture, wherein the assembly fixture comprises an anisotropicthermal expansion property, and wherein the anisotropic thermalexpansion property is based on an orientation of the plurality ofreinforcement fibers within the assembly fixture; fastening a pluralityof components of a composite structure in an assembly fixture, whereinthe assembly fixture comprises a plurality of strands of afiber-reinforced thermoplastic material, wherein the fiber-reinforcedthermoplastic material comprises a thermoplastic embedded with aplurality of reinforcement fibers, wherein the plurality ofreinforcement fibers is aligned within each strand of the plurality ofstrands, and wherein the assembly fixture further comprises ananisotropic thermal expansion property based on an orientation of theplurality of reinforcement fibers within the assembly fixture; andheating the assembly fixture in an autoclave to bond the plurality ofcomponents of the composite structure; wherein the anisotropic thermalexpansion property of the assembly fixture comprises a coefficient ofthermal expansion for a length dimension of the assembly fixture that issmaller than a coefficient of thermal expansion for a length dimensionof the composite structure.